Boron nitride is a thermally stable, highly refractory material of increasing commercial significance. Typically, boron nitride is produced by processes wherein boric acid is utilized as the boron source of reaction compositions. Suggested processes for producing boron nitride from boric acid are described in U.S. Pat. Nos. 2,922,699, 3,241,918, and 3,261,667 as well as in British Pat. Nos. 874,166, 874,165, and 1,241,206.
Processes wherein boron nitride is generated from boron oxide, as opposed to boric acid, are of special interest since per unit weight of reactant materials boron oxide contains more boron than boric acid, the additional boron theoretically making higher product yields of boron nitride possible. However, processes wherein boron oxide is a reactant are generally complicated, cumbersome procedures. For example:
U.S. Pat. No. 3,208,824 discloses a process for producing boron nitride using as a boron source compounds such as B.sub.2 O.sub.3, H.sub.3 BO.sub.3, and alkali and alkaline earth metal borates. The process involves mixing a selected boron-containing compound and an inert solid diluent with water to form a pasty mass, extruding the pasty mass into small particles, drying the particles at a sufficiently high temperature to vaporize the water, and nitriding the dried particles with ammonia.
U.S. Pat. No. 3,232,706 discloses a process wherein boron oxide is introduced into the high temperature reaction zone of an arc furnace, vaporized and reacted with a reactive nitrogenous gas such as ammonia to produce boron nitride. The process requires the use of a specialized high temperature furnace wherein a high temperature reaction zone is produced by utilizing one or more non-consumable electrodes to generate a high temperature arc.
U.S. Pat. No. 3,429,722 discloses a process for producing boron nitride fibers wherein a boron oxide fiber having a maximum diameter of about 30 microns is heated at a temperature rise between 25.degree..degree. C. per hour and 5,000.degree. C. per hour up to a final temperature between 300.degree. C. and 1500.degree. C. in a current of ammonia. The patent discloses that particle size is critical to obtaining substantially complete conversion of B.sub.2 O.sub.3 to born nitride (column 2, line 72 to column 3, line 11). U.S. Pat. No. 3,429,722 further discloses that by heating the oxide in ammonia, the boron oxide fiber is provided with a protective shield of a nitrogen and hydrogen containing compound which retains the fibrous form of the B.sub.2 O.sub.3 reactant and prevents fusion of the fiber.
U.S. Pat. No. 4,130,631 discloses a process for forming a shaped article of fused boron nitride fibers which comprises forming a shaped article from a blend of boron oxide fibers and boric acid, heating the article in an anyhydrous gas to a temperature above the melting temperature of the boric acid for a time sufficient to melt some of the boric acid to the boron oxide fibers and, either simultaneously with or subsequent thereto, heating the article in an ammonia atmosphere to convert the boron oxide and boric acid to boron nitride. U.S. Pat. No. 4,130,631 requires that the article be heated in such a manner that the boron oxide fibers are not destroyed by melting or decomposition.
It is further known that boron oxide may be reacted with ammonia in the presence of a tertiary calcium phosphate carrier at temperatures of about 900.degree. C. to produce boron nitride. (See Gmelin's Handbuch der anorganischen Chemie, supplement to 8th edition, vol. 13, part 1, pages 1-6 (Springer Verlag, 1974) as cited in U.S. Pat. No. 4,107,276). In this process the tertiary calcium phosphate carrier distributes the boron oxide in a thin sheet, providing a larger surface for oxide reaction with ammonia thereby reducing the tendency of boron oxide to form large unreactive lumps or agglomerations. However, such a reaction requires the use of extremely pure reactants and employs various heating, drying, homogenation and filtration steps.
In general, the above described processes are time-consuming, multi-step procedures, which produce commercially unattractive yields of boron nitride, require the use of boron oxide in fibrous form or utilize highly specialized equipment. Moreover, the processes typically require the use of an ammonia nitriding atmosphere, which for environmental and toxicological reasons is oftentimes objectionable.
Accordingly, it is an aspect of this invention to provide a simplified, high-yield process for producing boron nitride directly from particulate boron oxide. It is a further aspect of this invention to produce boron nitride by a process wherein nitrogen may be substituted for ammonia as a nitriding agent.